Thin film deposition techniques require precise control of deposition parameters to produce the complex structures demanded for current and next generation applications. For example, molecular beam epitaxy (MBE) is a versatile technique for depositing single-crystal semiconducting, insulating, or metallic materials used in fabricating state-of-the-art electronic and opto-electronic devices. The advantages of MBE over other growth techniques include the ability to produce high purity materials with controlled composition, layer thickness, dopant concentrations, and structure. MBE growth is achieved by directing the output of effusion cells onto a heated substrate in an ultra-high vacuum (UHV) chamber. An effusion cell's output includes atoms and molecules of the desired growth and doping constituents that are to be deposited on the substrate.
MBE practitioners have long sought real-time, or in-situ, control of growth rates and composition of the deposited material. Although MBE offers the potential for growth of device structures with atomic layer precision, current so-called “dead reckoning” methods employed for controlling effusion cell fluxes place limitations on the extent to which the desired composition, thickness and layer uniformity can be achieved. In-situ monitoring and control of these parameters hold the keys to achieving higher accuracy in attaining target growth structures and improved run-to-run reproducibility. Current generation MBE machines rely on pre-growth calibrations such as ion gauge pressure flux measurements, quartz crystal monitors (QCM) or reflection high energy electron diffraction (RHEED) oscillations to determine proper flux conditions. These methods are time-consuming, provide no real-time feedback, and are only marginally accurate when growing demanding structures. These problems directly increase device manufacturing costs.
A few methods of providing true real-time feedback have been proposed. For example, one such class of techniques is based on optical flux monitoring (OFM). OFM detects changes in transmitted light intensity from absorption by the atoms emitted from the effusion cell. However, transmission changes in the OFM system that are not due to flux changes of the atomic beam are often detected as well. In addition, the atomic flux is measured near the surface of the substrate. This geometry is not optimal for several reasons. The foremost problem is that the transmitted light intensity is affected by atoms reflected or desorbed from the substrate surface. Monitoring atomic flux at close proximity to a substrate allows double counting of atoms if they have non-unity sticking coefficients. Also, the atomic flux is not monitored continuously. With these OFM approaches, there is no signal when the effusion cell shutter is closed. This necessitates “dead-reckoning” based upon effusion cell temperature data to infer initial flux conditions after the shutter is opened. An additional shortcoming of existing OFM detection geometries is that, owing to the divergent nature of the effusive source, an OFM signal is measured just above the substrate, where the number of atoms crossing the optical path is lowest. Finally, implementing OFM on existing MBE machines requires substantial re-working of the vacuum chamber and cryopanels.
Accordingly, there remains a need for improved methods and devices for measuring flux from effusion cells in MBE systems. It would be particularly advantageous if such methods and devices could be used with other thin film depositions systems and techniques. It is to the provision of such methods and devices that the various embodiments of the present invention are directed. More specifically, it is to the provision of methods and devices for the in-situ measurement of flux from precursor sources in a variety of thin film deposition systems, as well as the associated thin film depositions systems employing these methods and devices, that the various embodiments of the present invention are directed.